Field of the Invention
The present invention relates to a die cushion apparatus of a press machine and a die cushion controlling method, particularly, to a technique of raising and lowering a cushion pad using separately-controllable multiple driving shafts.
Description of the Related Art
A general die cushion apparatus of a press machine performs a die cushion force control on a cushion pad during a stroke from a die cushion standby position to a press bottom dead center while a lower die (blank holder) and an upper die (dies) mounted to a slide of the press machine via a material are in tightly contact with each other.
As shown in FIG. 10A, in a state where a cushion pad 2 is held at the standby position, a plane face of the cushion pad 2 (blank holding plate 206 supported via cushion pins 1) is disposed in parallel with a slide 104 (plane face to which an upper die 202 is mounted) of the press machine. Note that it is obvious for those skilled in the art who are manufacturers and users of such an apparatus to set and adjust the cushion pad 2 to be held at the standby position in parallel with the slide 104.
In the above die cushion force control, the cushion pad often becomes inclined in accordance with the slide (plane face to which the dies are mounted) having greater rigid via the die and the material because a dimension of each die (die+material thickness+blank holder+cushion pin length) becomes different, which results from the degree of shim adjustment on various portions of the die, and local variation in plate thickness of the material (particularly, significant variation in plate thickness of a tailor welded blank material), or in the case of forming the material using two or more types of dies between a single slide of the press machine and a single cushion pad.
If the die cushion force is controlled as shown in FIG. 10B, the cushion pad 2 becomes inclined in accordance with inclination of the lower faces of the dies (left and right upper dies 202) mounted to the slide 104.
FIG. 11 is a graph showing a time course of a die cushion forces L, R generated on the left and right driving shafts that chiefly support the cushion pad 2 during the die cushion force control. As shown in this drawing, at the start of the die cushion force control, the right die having a longer length initially collides, and the right die cushion force R starts up at the time t1. The left die collides following the right die, so that the left die cushion force L starts up at the time t2 behind with the time t1.
As a conventional die cushion apparatus for controlling the cushion pad through multiple driving shafts, a die cushion apparatus described in Japanese Patent Application Laid-Open No. 2007-136500 has been known.
The die cushion apparatus described in Japanese Patent Application Laid-Open No. 2007-136500 includes multiple die cushion mechanisms (screw nut mechanisms) having driving shafts respectively connected to a cushion pad, multiple servo motors for respectively driving the multiple die cushion mechanisms, and a control unit for controlling multiple servo motors, wherein the control unit controls currents supplied to each servo motor based on the load generated on a cushion pad, thereby controlling the die cushion force. A position in the vertical direction of the cushion pad is measured by a linear scale mechanism, and information regarding the measured position of the cushion pad is outputted to the control unit.